flutter - 取消 future 超时的优雅方式

Question

在这些情况下，我需要尝试某事一段时间或多次，但不会太久。 Future 的 .timeout 属性看起来是一个自然的选择。我了解到，原始的 Future 即使在超时后仍继续运行，例如:

  Future<void> trySomething() async {
    while (true) {
      print('trying something...');
      bool successful = false; // exit if done
      if (successful) {
        return;
      }
      await Future.delayed(const Duration(seconds: 1));
    }
  }

  trySomething().timeout(const Duration(seconds: 10), onTimeout: () {
    print('try something else...');
  });

以上代码产生以下输出:

I/flutter (26426): trying something...
I/flutter (26426): trying something...
I/flutter (26426): trying something...
I/flutter (26426): trying something...
I/flutter (26426): trying something...
I/flutter (26426): trying something...
I/flutter (26426): trying something...
I/flutter (26426): trying something...
I/flutter (26426): trying something...
I/flutter (26426): try something else...
I/flutter (26426): trying something...
I/flutter (26426): trying something...
I/flutter (26426): trying something...
I/flutter (26426): trying something...
I/flutter (26426): trying something...
I/flutter (26426): trying something...
I/flutter (26426): trying something...
I/flutter (26426): trying something...
I/flutter (26426): trying something...
I/flutter (26426): trying something...

我不太优雅的解决方法是使用外部标志变量向 main future 中的进程指示不再需要其服务:

  bool youAreTooLate = false;
  Future<void> trySomething() async {
    while (true) {
      if (youAreTooLate) {
        return;
      }
      print('trying something...');
      bool successful = false; // exit if done
      if (successful) {
        return;
      }
      await Future.delayed(const Duration(seconds: 1));
    }
  }


  trySomething().timeout(const Duration(seconds: 10), onTimeout: () {
    youAreTooLate = true;
    print('try something else...');
  });

这产生了下面的预期结果，但并没有让我内心变得那么温暖和模糊。

I/flutter (26426): trying something...
I/flutter (26426): trying something...
I/flutter (26426): trying something...
I/flutter (26426): trying something...
I/flutter (26426): trying something...
I/flutter (26426): trying something...
I/flutter (26426): trying something...
I/flutter (26426): trying something...
I/flutter (26426): trying something...
I/flutter (26426): try something else...

有没有更好的方法来处理这种情况？

Answer 1

如果您只是想重复做一些有延迟的事情，您可以使用 Timer 来实现这一点，同时保持取消它的能力。

final timer = Timer.periodic(const Duration(seconds: 1), (t) {
  // Do repeated work here.

  if (shouldCancel) {
    // This is the 'timer' object.
    t.cancel();
  }
});

Future.delayed(const Duration(seconds: 10)).then((_) => timer.cancel());

如果您希望真正等待计时器内部的工作完成后再重试，您可以这样做:

Timer timer;

doWork() async {
  // Do your work here.
  // ...

  if (retry) {
    timer = Timer(const Duration(seconds: 1), doWork);
  }
}

Future.delayed(const Duration(seconds: 10)).then((_) => timer?.cancel());

这应该可行，但如果您已经有一个未完成的 Future，则无法实际取消它。